Control apparatus



May 22, 1945. w L SHAFFERI 2,376,513

CONTROL AIFPARAIUS Filed Oct. 9, 1942 3 Sheets-Sheet 1 FIGQI.

INVENTOR. WILLIAM L.SHAFFER.

ATTORNEY.

y w. SHAFFER 2,376,513

CONTROL APPARATUS Filed Oct. 9, 1942 3 Sheets-Sheet 2 FIG. 2.

ANVEN TOR. WILLIAM L SHAFFER ATTORNEY.

May 22, 1945. w 5|- F R 2,376,513

CONTROL APPARATUS Filed. Oct. 9, 1942 3 Sheets-Sheet '5 INVENTOR. WILLIAM L. SHAFFER.

- v ATTORNEY.

Patented Ma 22, 1945 I 2,376,513 'coN'rn'oL APPARATUS William L. Shaffer, Philadelphia la assignor to The Brown Instrument Company, Philadelphia, Pa., a corporation'of Pennsylvania Application October 9, 1942, Serial No. 461,413 19 Claims. .(Cl; 172-239) My present invention relates to-electrical control apparatus and in particular to electrical control apparatus designed to effect suitable corrective adjustments in the value of a quantity being controlled, on departure of the controlled quantity from a predetermined value, without creating an objectionable tendency to unstable control or' hunting.

It is a general object of my invention to provide electrical control apparatus which is capable of producing a corrective adjustment in a controlling medium, upon change in the value of a variable quantity under control, torestore the latter to the value desired in the shortest time possible while avoiding the tendency to hunting.

A more specific object of my invention is to provide such electrical'control apparatus utilizing an alternating current control network and embodying physically stationary means for compensating for variations in the characteristics of the quantity being controlled. 1

trolled, or to any other variable conditions. on a change in an operating condition, such a change in the B. t. u. content of the fuel supplied to a furnace or to a change in the furnace load, the furnace temperature will tend to vary but due to the heat inertia of the furnace, the change in the operating condition may have-been in existence for some time before it results in a temperature change which is detected by the apparatus utilized for the purpose of maintaining the furnace temperature at a desired value. When thereafter a correction in the amount of fuel supplied to the furnace is madeby the control apparatus, such correction will not be efiective immediately to restorethe desired furnace temperature. This lag also is due to the heat inertia of the furnace. In

It is a particular object of the invention to provide such apparatus in which the compensation for variations in the characteristics of the quantity being controlled is effected at a. rate proportional'to the extent of departure of the quantity from the desired value, and moreover, is uniform for a constant departure.

A further and more specific object of my invention is-to provide such electrical control" appa- 1 ratus utilizing an alternating current control network and embodying physically stationary means for producing a disprbportionatelystrong corrective eifect in the supply of the controlling medium during the initial stages of a departure of the quantity being controlled from the-desired valve while'avo'iding the tendency to hunting which would ordinariiy resultfrom such strong initial corrective effect.

Electrically operated controllers embodying the features of my invention may takewidely dif -ierent forms and are adapted for use for many diiferent p may be used whenever it is d ir'able to produce a control effect in response to a change in a control condition or quantity, such for example, as temperature, pressure, no liquid level, etc. which tends-to vary as a result of the control effect in rdance with the present invention, suitable provisions are made to prevent variations in the .quantitybeing controlled. Such variations maybe due to changes in the effect of the controlling medium, or agent, to changes in'the characteristics or amount or the quantity being conaddition, if a sumciently large correction is made to restore the furnace temperature to the desired value within a reasonably short time and that correction is maintained until that value is reached, the furnace temperature will tend to overshoot the desired value and subsequent corrective adjustments 'willresult in hunting or oscillation of the furnace temperature about that value.

In its broad aspects, therefore, a primary object of my invention is'to provide electrical control apparatus which is adapted to prevent such hunting or oscillation and is capable of effecting control or regulation at an even given value.

The preferred form of my invention includes ment whereby on a change in an operating condition, such for example, as a change in the load on a furnace which is being controlled to minimize variations in the, furnace temperature, the

tendency of. the furnace load or other operating condition change to increase or decrease the furnace temperature or other controlling condition urpdses- -In 7. general, however, they may be neutralized.

The preferred form of the invention also includes means for effecting an adjustment in the rate of reset whereby on a change in the furnace load or another analogous operating condition; the compensating adjustment necessary to the maintenance of the approximately constant value of the furnace temperature or other quantity under control may be varied to the end that the time required for full compensation of the operating condition may be reduced to the practical minimum possible without risk of objectionable hunting. 3 a a The preferred embodiment of my invention also drawings and descriptive motor is operating with the .instrument is disclosed December 1,1941.

' as is-required to includes provisions for eifecting a relatively large initial corrective adjustment in the amount of the controlling medium supplied cludes provisions for readily eflecting a so-called justed in response to a given change in the furnace temperature or other control quantity may be varied.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages, and specific objects attained with its use, reference should be had to the accompanying matter in which I have illustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. 1 is a diagrammatic representation of one embodiment of my invention;

Fig. 2 is a diagrammatic representation of the preferred form of my invention; and a modiflcationof the arrange- Fig. 3 illustrates ment of Fig. 2.

In Fig. there is illustrated, more or less diwith two armatures which are on the motor shaft and. field windings individual to each of the armatures.

displaced 6 and I c When the field winding- 6 energized, the direction of rotation is such as-to increase the opening of the to raise the The temperature of the furnace l is indicated by a pyrometer 8 which may be of any suitable type for indicating the E. M. F. of a thermocouple shown at 9. The pyrometer l may, for example, take the form of the mechanical or "conversion type potentiometer control instruments .ma'nuinthe walter P w111s Serial No. 421,173

application which was filed on tact and to which is the adjustment of valve 3 and thereby the fuel admission in-accordance with the extent to which the temperature of the furnace has departed from the desired temperature and also the duration of that departure. To these ends provisions eter 8 from the position corresponding to the desired temperature. Such initial operation of the valve 3 following a furnace temperature change is commonly known as a follow-up adjustment. The movable element I0 of the pyrometer 8 is the member of the potentiometer control instrument which carries the slidewire conattached the indicating pointer orpen when the instrument is of the indicating or recording type.

Thereafter the valve 3 is given additional opening or closing adjustments as required to return the furnace temperaturerto the desired temperature thus preventing drop in the furnace temperature. Such additional valve opening or clospotentiometer control" second slide contact It -In order to control the of the valve 3 v maintain the desired temperature, it is necessary that the dication of the pyrometer I. In addition. in order to avoid overshooting or "hunting"'of the furnace temperature about the temperature it is desired to maintain, it is desirable to control ing adjustments are known as resetting" or compensating adjustments and are provided to compensate for a change in furnace load or other condition which tends to vary the value of the controlling condition which the apparatus tends to. maintain. In general, such resetting adjust- "40 ature lag is produced in different ways, for example, by the inertia of ments in controlling the operation of machinery, and by the fluidstor'age capacity of portions of a fluid distribution system in which fluid pressures or rates of flow are controlled.-

The provisions. ing the position of the valve {to correspond proportionally with the position of the potentiometer slidewire contact and for thereafter effecting the desired resetting adjustments include a remote control system It for transmitting the mechanical movement of potentiometer instrument. The remote control system II includes a controlnetw'ork l2 inywhich a pair of 'slidewire removable element II but is electrically insulated therefrom contacts the slidewire resistance I! at a=position dependent upon the temperature-indication of the ,pyroineter I. A similarly contacts the slidewire resistance J4 at a position dependent uponthe opening of the I mechanism but is electrically insulated therefrom.

The control V ternatlng current from. alternating ply conductors L and I? through a suitable step-down transformer cu rent m the medium of if hunting is to be referred to for initially cans-I the movable element ll network l2 isenergiz'ed with al- I1. thep mary ply lines L winding l8 of which is connected to the supplyconductors L and L and the secondary winding [9 of which is connected by conductors 20 and 2| to the network 12. In accordance with the arrangement of Fig. 1 the alternating potential drop between the sliding contact l5 and the right end of slidewire resistance I3 is compared to the alternating potential drop between the sliding contact I6 and the right end of resistance 14. These alternating potential drops are combetween the right end of resistance l3 and'the contact I5 is impressed on the input circuit of an electronic valve 23 and the alternating potential drop between the right end of resistance l4 and contact 16 is utilized through eleotrOnic.valVes 24, 25 and 26, which are interconnected as shown and whichoperate in a manner described in detail hereinafter, to control the potential impressed on the input circuit of an electronic valve 21, The valves 23 and 21, although shown in sep- I arate envelopes, may be contained in the same envelope, if desired.- The electronic valves 24, 25 and 26 are interposed between the slidewire resistance l4 and the input circuit of electronic valve 21 for the purpose of efiecting the desired follow-up and resetting adjustments of the 8P1 paratus.

The electronic valves 23 and 21 are triodes and each includes anode; control electrode, cathode and heater filament elements. The cathodes of valves 23 and 21 are connected together and through a biasing resistance 28 which is shunted by a condenser 29 to a conductor 36 which is grounded at 3i and right end terminals ofthe slidewire resistances l3 and l4. 4

The anode of valve 23 is connected to one end terminal of the primary winding 32 of a transformer 33having a secondary winding 34, and the anode of valve 21 is connected to the other end terminal of winding 32. A center tap on the transformer primary winding 32 is connected by a conductor 35 to the positive output terminal of a filter 36 the input terminals of which are connected to the output terminals of a full wave rectifier 31. The rectifier 31 includes a full wave I rectifier valve 38 having a pair of anodes and an associated filament type cathode. The anodes of valve 38 are connected to the opposite terminals of the high voltage secondary winding 39 of a transformer 48 having a winding line voltage. primary 4| connected to alternating current supand L and also having low voltage secondary windings 42 and '43. The secondary winding 42 is connected to the filament cathode of valve for supplying energizing current to the filament cathode and is provided with a center tap which is connected to the positive input terminal of filter 36. A center tap on the secondary winding 36 is connected to the negative input terminal of the filter 36 and the negative output terminal of the latter is connected ranged to regulate the reversible motor 5 for opis also connected to the through an adjustable resistance 44 to the grounded conductor 33.

Thus, the anode circuits of valves 23 and 21 are arranged to be energized with direct current voltage. The'anode circuit of valve 23 may be traced from the positive terminal of the filter 36 through the conductor 36 to the center tap on the transformer primary winding 32, through the upperhalf of winding 32 'to the anode of valve 23, the cathode thereof, the parallel connected resistance 28 and condenser 23,'and conductor 30 to the negative output terminal of filter 36. The anode circuit of valve 21 may be traced from the positive output terminal ofvfilter 36 through conductor 35 to the center ta on the primary a winding 32, through the lower half of the latter to the anode of valve 21, the cathodethereof. the parallel connected resistance 28 and condenser 23', and conductor to the negative terminal offilter 36.

Energizing current is supplied the heater filaments of the valves 23,.and 21 from the low voltage transformer secondary winding 43.

The input circuit of valve 23 maybe traced from the cathode thereof through the parallel connected resistance 28 and'condenser 29 to the conductor 36, through the right end portion of slidewire resistance l3 to the contact l5, and a conductor I5 to the control electrode of valve 23.. Thus, the alternating contact [5 and the right end of slidewire resistance l3-is impressed on the input circuit of valve 23, being superimposed on the unidirectional potential impressed thereon by the biasing resistance 28 and condenser 29. The input circuit of valve 21 may be traced from the cathode-of this valve through the biasing resistance 28 and condenser 29 to the conductor 36, a slidewire resistance 45 and a sliding contact 46 which is in engagement with resistance 45 to the control electrode of valve 21. An alternating potential is impressed on the slidewire resistance 45 from the secondary winding. 41 of a transformer 48 having a primary winding '49 which is connected in the anode circuit of valve 26. The magnitude of this alternating potential impressed across resistance '45, and a variable portion of which is impressed cnithe in ut circuit of valve 21 depending upon the position of contact 46 along resistance 45, is determined byithe position of sliding contact l6 along resistance l4'in a mandenser 29.

the induction of an ner explained in detail hereinafter.

perimposed on the unidlrectionalbiasing potential derived from the resistance 28 and con- The application of an alternating potential on the input circuits of each of the valves 23 and 21 causes a flow of pulsating current in the anode circuits of each of these valves and thereby through the opposite halves of the center tapped transformer primary winding 32. The pulsating current flow through the upper half of primary winding 32 causes the induction of an alternatin the'transformer sec- 34 and the pulsating current flow half of'the winding 32 causes alternating volts-8e of the the secondary winding 84.

ondary winding through the lower opposite phase in When the pulsating opposite halves of the primary winding 82 are exactly equal and precisely inphase with each other, the fluxes established in the transformer 33 by each of thesecurrent flows will cancel each potential between the The input circuit of valve 21, therefore, also has an alternating potential impressed thereon which is sucurrent flows through the II are at the, same duced in the secondary winding 34. Predominance of pulsating current fiow in the upper or lower half of the'primary winding 32, however, will result in the induction of an alternatin voltage of one phase or of the opposite phase in the are connected to and receive energizing current from thetransformer secondary winding 43.

The cathodes of triodes 5i and 52 are connected together and through a biasing resistance 53 which is shunted by a condenser 54 to the grounded conductor 30. Theinput circuit of triode ll may be traced from the cathode thereof, through the parallel connected resistance 53 and condenser 54 to the conductor 34,-the portion of slidewire resistance I4 to the right of sliding contact II and from the latter through a conductor II- to the control electrode'ot valve ii. The input circuit oi triode 52 may be traced from the cathode of this triode through resistance is and condenser 54 in parallel, to conductor II, the right half or slidewire resistance .|4,'and conductor II which is connected at-one end to a contact I! in slideable engagement with the resistance l4 and at its other' end to the control electrode oi valvel2. Thus. the alternating potential between the sliding contact I and the rightend 2,876,618 v other whereby no alternating voltage will be proof resistance l4..is impressed on the input circult oi. triodell, andthe alternating potential on slidewire resistance l4 to .the'right of contact 51 is impressed on the input circuit of triode l2.

In eachvcase the impressed alternating current.

is superimposed on the unidirectional bias potential provided by the parallel-connected resistanceand'condenser 54. when the contacts II and position along the slidewire resistance l4, the combined alternating and unidirectional potentidl impressed on the input circult of triode 5| will be exactly the same as that impressed on the input circuit of the triode". Upon operation of motor I and consequent move-. ment of contact II in one direction or the other from thisposition; however, the alternating compcnent of the potential impressed on the input circuit of triode 6i will be increased or decreased relatively to that impressed on the input circuit of triode 82. Stateddiflerently, the alternating potential impressed on the input circuit or triode I2 is fixed in magnitude while that impressed on the input circuit or 'triode II is linearly representative oi the position of valve 3 from a predetermined position.

Direct current voltage is applied to the anode circuits of both triodes II and 52 from the .iull wave rectifier l1 and filter 34 through a circuit which may be traced from the'positive terminal of filter I! through a conductor I! to the point oi engagement of a pair of resistances II and Cl. From that point the anode circuit of triode II is traced through the resistance ll, the anode to cathode resistance oi-triode Ii, the parallel connected'recistance II and condenser l4, conductor 30, and resistance 44 to the negative terminal of filter Ii. The anode circuit oi. triode 82 is traced from the. point orengagement oi resistances 59 and 60 through resistance 88, the anode of triode 52, the cathode, resistance 53 and condenser 54'in parallel, and conductor 30 and resistance 44 to the negative terminal or filter l6. 7 Since the potentials impressed on the input circuits of triodes Si and 52 have an alternating component, the current flow through the resistances 59 and 80 in the anode circuits will be pulsating, the magnitude of the pulsating potential drop across resistance 59 being determined by the position of contact I8 along resistance l4 and that across resistance 60 being determined by the position of contact 51 along resistance II. The pulsating potential drop across resistance is fixed. and that across resistance 52 isrlinearly representative of the opening of the valve 1.

Resistances 58 and 60 are coupled to the electronic valve 25 by condensers Bi and 62 and resistances $3 and .64 across the latter of which an alternating potential of one phase or the opposite phase is produced depending upon the relative magnitude oi, the pulsating potential drops produced across resistances 59 and 80. Valve 25, as

, shown, includes two inversely connected diode elements 5 and 66 within one envelope. Each diode includes an anode, a cathode, and a heater filament to which energizing current .is supplied by the transformer secondary winding 43. The remote ends of resistances 59 and iii are connected by a separate QiIdenser GI and 62 to the anode and the cathode, respectively, of an associated diode 85 and il. The resistances 63 and 84 are connected in series between the anode and cathode of the .diodes 6i and 48 and the point of engagement of the resistances i3 and is connected to the grounded conductor ill. The cathode oi diode 65- and anode of diode II are connected together and through a parallel connected resistance 4'! and condenser 84 to'the grounded conductor 3|.

Thus, the alternating potential drop produced across resistance fl is arranged to be impressed on the diode II and the parallel connected resistance l1 and condenser 68, and the alternating potential drop produced across resistance 84 is arranged to be. impressed 'on the diode I and the parallel connected resistance '1 and condenser when the alternating potential produced across I resistance 43 is greater than that produced across resistance 44' due 'to the'coupling of these resistances to the resistances 59 and 40, as is the case when the alternating potential impressed on the input circuit or triode ii is greater than that impressed on the input circuit of triode 52, a re-,-

sultant unidirectional 'potential drop having the, polarity torender the upper terminal positive will be produced across resistance 81 due to the rectifying action or the diodes 85 and 8. Conversely, when the alternating potential impressed on the resistance 3 is less than that produced across resistance 64, as is the case when the alternating potential impressed on the input circult of triode I2 is predominant, a resultant unidirectional potential drop will be produced across resistance '1 having the .polarity to render the upper end negative.

No potential drop will be'produced across re-- 'sistance 41 when the alternating potentials on the input circuits or both triodes SI and I2 are equal, which condition exists when the positions of sliding-contact ll and contact "along resistance l4 coincide. Upon movement or slidins produced acrossresistance 61 will be such as to render the upper terminal thereof positive. Movement of sliding contact IE to the right from that position, on the other hand, will result in apredominance of alternating potential on the input circuit of triode 52 andthereby in the production of a potential drop across resistance Bl of the polarity to render the upper terminal of the latter negative.

Rectifier. valve 25 in conjunction with elements 6|, 62, 63, 84, 67 and 68, therefore, operates to .compare and'convert the pulsating output po:

tentials of trlodes i and 52 to produce a unidirectional potential of polarity dependent upon the departure of valve 3 from a predetermined position (determined by the position of contact 51 along slidewire resistance [4) and of a magnitude proportional to the extent of the valve departure from that position.

-- The unidirectional potential drop so produced across resistance 61 is applied to the input circuit of the electronic valve 26 through a network including a variable condenser 69 anda resistance '16 which is provided with an adjustable tap H. This network operates to impress changes only in the potential drop across resistance 61 to the input circuit of valve 26 and may be traced from the upper end of resistance 61 through condenser 66, resistance I0 and conductor to the lower terminal or resistance 61. The lower terminal of resistance 10 is connected through a bias resistance 12 which is shunted by a condenser 13 to the cathode of valve and the tap"'|l is connect'edto the control electrode thereof. When no potential 'dropexists across resistance 61. no

current will flow through condenser 69 and re- I 'HL an'd therefore, underthis condition no potiQtiaI is impressed on the input circuit of valve 26. Upon the appearance of a potential drop of one polarity across resistance 61, however, a charging current will flow throughresistance '16 to condenser 69 until the condenser is fully charged for the particular value of potential across resistance 61. The direction of chargin current flow through resistance I0 is determined by the polarity of the potential drop across re-- sistance 61 and is downward to render the upper terminal of: resistance 16 positive when the upper terminal of resistance 61 is positive, and is up- .ward to render the lower terminal of resistance .10 positive when the lower terminal of resistance 61 is positive. Thus,- the potential drop produced across resistance 10 is variable in'polarity and is of one polarity or the other dependent upon the polarity of thepotential across resistance 61 and also upon the relative. ma nitudes of the potentlals across resistance 61 and condenser .66. The

potential drop across resistance 10, in addition, is transitory since charging current flows through this resistance to produce the potential drop only so long as charging current flows into or out of the condenser]! at the then value of potential drop across resistance. This operation is utilized in a mannerto be explained to produce the desired resetting adjustments of the system. Valve 26 is a variable-mu pentode such, for

example, as the type known and sold commerclall'y as the 6F? and includes an anode, suppressor, screen and control electrodes, a cathode and a heater filament to the latter 'of which plied valve 26 from the full wave rectifier 31 and filter as through a circuit which may be traced from the positive terminal of filter 36 through conductor 35, the primary winding 49 of transformer 58, the anode to cathode resistance of valve 26, theparallel connected resistance '32 and condenser 13, and conductor 30 and resistance 44 to the negative terminal of filter 36. Screen voltage is obtained by connecting the screen through a. resistance M to the positive terminal of filter 36. A condenser I5 is also employed to connect the screen to the conductor 36 and throughresistance 44 to the negative terminal of filter 36.

The input circuitof valve 25includes, in addition to the network comprising elementsli'l, 6t, 69, Ill, 1!, l2 and 13, a branch which may be traced from the control electrode through a conformer H, the circuit branch just traced operates to impress an alternating potential on the input. circuit-oi. the valve 26. The magnitude of this alternating potential is determined by the position of the sliding contact 18 along the length of the "resistance 19. As is explained more fully hereinafter, adjustment of contact 18 along resistance 16 operates to vary the control point of the system, or in other words, the temperature of the furnace I which the apparatus seeks to maintain. a

The application of this alternating potential on the input circuit of valve 26 causes a pulsating current flow through the transformer pri mary winding 46 and thereby the induction of an alternating potential in the transformer secondary winding 41. This induced alternating po tential is impressed across resistance 45 and a portion of the alternating potential across the latter is impressed onthe input circuit of valve 21 as previously explained.

The magnitude of the alternating potential inducedin the transformer secondary winding 41 and thereby impressedlon the inputcircuit of triode 21 is determined primarily b the magnitude of the alternating potential impressed on the input circuit of the valve 26, but is arranged to be varied in accordance with the'polarity and magnitude of the unidirectional potential impressed on the input circuit of valve 26 from the resistance 16. This result is obtained by virtue ofthe operation of the valve 26 which, as already noted, is a variable-mu pentode. The amplification factor of such valves is a function of the negative bias applied to the control electrode,

' and more particularly, decreasesas the negative bias is increased. The cathode biasing resistance 12 and condenser 13 operate normally to maintain a predetermined negative bias on the control electrode of valve 26, and accordingly,

when. no potential drop is impressed on the input circuit of valve 26 from the resistance 10,

the amplification factor of valve 26' isa predetermined value. At this amplification factor the circuit ofvalve 26 from the portion of resistance 19 to the right of contact I8 causes a pulsating tion of an alternating potential of predetermined magnitude in the transformer secondary winding current flow of predetermined magnitude in the v output circuit of valve 26 and thereby the induc- It is noted that the sliding contact 46 is 'so adjusted along the length of resistance 45 that the alternating potential then impressed on the inputcircuit of valve 21 is exactly the same as. that impressed on the input circuit of valve 23 when the sliding contact I is at aposition along the length of resistance I3 corresponding to the temperature it is desired to maintain in the furnace I.

when a unidirectional potential is produced across the resistance as a. result of a change in the potential drop across resistance 51 and thereby as a result of a change in the position of sliding contact I6 along the length ofresistance I4, however, the amplification of. the valve 20 is varied in one direction or the other and to an extent depending upon the direction and the extent of the change to produce a corresponding change in the alternating potential induced in the transformer secondary winding 41 and thereby applied to the input circuit of valve 21. The alternating potential applied to the input circuit of valve 21 is thus varied, in response to a change in the position of sliding contact I6 along resistance I4, to balance the variations in alternating potential applied to the input circuit of'valve 23 produced by a change in position of the sliding contact I 5 along slidewire resistance I3 and thereby to reduce to zero the alternating voltage induced in the transformer secondary winding 34 as a result of such change in position of contact I! along slidewire resistance I8. Because of the 'ment elements.

charging action of condenser 69 in the input circult of the valve 26, however, the change in negative bias potential on the input circuit of valve 20 diminishes after a time and as a result the When analternatingpotential of one phase or the other is induced in the transformer secondary winding 34, pulsating current of corresponding phase will fiow in the output circuit of valve 80 through the transformer primary winding 84 to cause the induction of an alternating voltage of corresponding phase in the transformer secondary winding 88. A condenser 00 is connected in parallel with the winding 80 for tuning the latter. The transformer secondary winding 86 isconnectedin a phase shifting bridge network 89 and comprises two arms thereof. A resistance 80 is connected in a third arm and a. condenser 9| is connected in the remaining arm.

Electronic valve 81 includes two triodes 92 and 93 in the same envelope. Each triode includes anode, control electrode, cathode, and heater fila- The heater filament elements are connected to and receive energizingcurrent from the transformer secondary winding 43.

Theinput circuits of triodes 92 and 90 are connected in parallel and is connected to the output terminals of the phase shifting bridge network 89. Specifically, the control electrodes of triodes 92 and 93 are connected to each other and to the point of engagement of the resistance 90 and condenser 8|. The cathodes of these triodes are connected together and are connected through the resistance 4 to the center tap o the transformer secondary winding 88.

Anode voltage is supplied the triodes 92. and 98 from the alternating current supply conductors L and L through the medium of a transformer at having a line voltage primary winding 95 connected to the supply conductors L and L and a high voltage center tapped secondary winding 86. The anode clrcuit'of triode 82 may be traced from the lower ,terminal of secondary winding 98, as seen in the drawings, through a conductor 91 in which the operating winding 00 of a relay-99 is inserted to the anode, the cathode, and a conductor I00 to the center tap on secondary winding 96. The anode circuit of triode I! may be traced from the upper terminal of secondary winding 86 through a conductor It'll in which the operating winding I02 of a-relay I03 is inserted to the anode, the cathode, and conductor M0 to the center tapon secondary winding 00.

input circuit of an electronic valve 80 which is cathode biasing resistance 8| which is shunted by a condenser 82 to the lower terminal-of transformer secondary winding 34, and through the latter to the control electrode of valve 80. Anode voltage is, supplied valve ,80 from the full wave rectifier 31 and filter 36 through a'circuit which may be traced from the positive terminal of filter 36 to a conductor 83 in which the primary winding 84 of a transformer is inserted, the anode to cathode resistance of valve 80, biasing. resistance BI and condenser 82 in parallel, and resistance 44 to the negative terminal of filter 38.

Transformer 85 is also provided with a center pp ond ry winding 86 which is connected I to the input terminalsof an electronic valve 81.

Relay 95 controls the operation of a switch arm I04 into and out of engagement with a fixed contact I05. When switch arm I04 engages contact 05, an energizing circuit is completed for field winding 6 of reversible motor 5. This circuit may be traced from the alternating current supply conductor L through a conductor I00 to the switch arm I04, contact I05, motor field winding 5 ends conductor III! to the supply'conductor L.

Relay I03 controls the operation of a switch arm I08 into and out of engagement with a fixed contact I09. When switch arm I08 engages contact I09, an energizing circuit is completed for field winding I of motor 5. This circuit may be traced from the supply conductor L through conductor I06 to the switch arm I08, contact I08, motor field winding 1 and conductor I01 to the supply-conductor L. It will be noted that since theanode circuits of triodes 92 and 93 are energized by opposite halves .of the transformer secondary winding 98 these circuitsare arranged to be conductive during alternate half cycles of the alternating current supplied by conductors L and I;

As is shown in the drawings, a limit switch no is connected between the motor field winding 8 and the switch I04, I05 controlled by relay 30, and a limit switch III is connected between the motor field winding I and the switch I00, I09 controlled by relay I03. Limit switches H and I II are operated by a suitable member connected to the motor shaft and are provided to interrupt the operation of motor when the latter has operated to the extreme limits of adjustment of fuel valve 3.

When no alternating potential is induced in the transformer secondary winding 88, the control electrodes of triodes 92 and 93 will be negatively biased by the unidirectional potential drop. produced across resistance 44 by the flow of current therethrough from the output circuits of valves 23, 26, 21 and 80 suiliciently to maintain the current 'flow through relay windings 98 and I02 below the normal operating level of the relays, and consequently, the switch arms I04 and M8 will then be out of engagement with their associated contacts I05 and I09 To insure this result the switch arms I04 and I00 are preferably biased by spring or other'means not shown out of engagement with their associated contacts.

Upon the induction of an alternating potential of one phase in the transformersecondary tacts I5 and I6 at the center of their associated slidewire resistances, the alternating potential impressed on the input circuit of triode 2'! from th transformer secondary winding 41 and resistance 45 will be exactly the same as that impressed on the input circuit of triode 23 from the portion of slidewire resistance I3 to the right of sliding contact l5. Under this condition no resultant alternating potential will be produced in the transformer secondary windings 34 and 86, and therefore, the relays 99 and I03 will both be deenergized and the energizing circuits to motor field windings B and I both open whereby winding 86, the control electrodes of triodes 92 and 53 will be driven less negative or even positive depending upon the magnitude of the in-- duced alternating potential during the half cycle that the anode of triode 82 is positive. This will result in an increase in the energizing'current flow through the winding 98 of relay 99 and thereby operation of switch arm I04 into engagement with contact M5 to close the energizing circuit to motor field winding 6. Similarly, when an alternating potential of opposite phase is induced in the transformer secondary winding 8 the control electrodes of triodes 92 and 93 will be driven less negative or even positive durin the half cycle that the anode of triode 93 is positive, thus effecting an increase in the energizing current flow through the relay winding I02 and thereby operationyof switch arm I08 into engagement with contact I09, and consequently. closur of the energizing circuit to motor field winding I. I

The phase shifting network 83 is provided to cause the alternating potential impressed on the control electrodes of the triodesv92 and 93 to b exactly in phase with the alternating potential impressed on the anodes of these triodes by the transformer secondary winding 96 and the components 90 and 9| of the network 89 are chosen so as to attain this result. Such a phase shifting network is required when it is desired. to have the relays 99 and I03 normally deenergized. The tuning condensers and 88 introduce some shift in phase in the alternating potential derived from the transformer secondary winding 34 and impressed on the input circuits of the triodes 92 7 and 93 relatively to the alternating potentials im-,

pressed on the anode circuits of these triodes. If such an alternating potential is impressed on the input circuits of the-triodes 92 and 03.-an increase in anode current through each triode and to each relay winding will be effected whereby both relays will tend to operate regardless of the phase of the impressed alternating potential. This difficulty is overcome by providing the phase shifting network 89 to compensate for such phase.

shifts.

- With the furnace operating condition such that the control network I2 has been balanced or stabilized for an appreciable period with the conpotential impressed on the input circuit oi'vthe the motor 5 will be at rest. In addition the alternating potential impressed on the input circuit of triode 5i from the portion of the slidewire resistance It to the right of contact Eel will be precisely the same as that impressed on the input circuit'of the triode 52iro'm the portion of resistance I4 to the right of contact Iii since the positions of contacts 51 and It along the resistance I4 then coincide. Therefore, ther will then be no current flow through the resistance- GI, nor through the resistance Ill, and furthermore, there will then be no charge on condenser 69. Any charge which may previously have ex isted on condenser 68 will have leaked off, the apparatus having been stabilized for an appreciable period, as noted, with no current flow through resistance 67. The negative bias impressed on the input circuit of the variable-mu pentode 26 will then she only that produced across th cathode biasing resistance I2, and accordingly, the. gain of the pentode 20 will be its normal, predetermined value whereby the alternating potential induced in the transformer secondary winding 41 will be of .such a value as to impress an alternating potential on the input circuit of triode 21 which is precisely of the same magnitude as the alternating potential impressed on the input circuit of triode 23 'whenthe contact I5 is in engagement with the resistance I3 at the center of the latter.

On a-change in the furnace heat output, for example, on an increase in the output followed by a prolonged period in which no further change in the rate of heat output occurs, the furnace temperature will decrease and subsequently the f sliding contact I5 will be adjusted bythe pyrometer 8 to the left along the length of resistance I3. Such adjustment of the contact It will .eflect an increase in the magnitude of the alternating triode 23 and thereby cause the inductionoi' a resultant alternating potential of the phase in v the transformer secondary winding 34 required to cause energization of the motor field winding 6 '-and consequently operation of the motor 5 'in the direction to produce an opening adjustment of the fuel valve 3 and an adjustment of the sliding contact" tothe left along the length of the resistance I4. Such adjustment of the contact I8 results inan increase in alternating potential onthe input circuit of triode SI and thereby in the production of a unidirectional potential drop across resistance 67 of the polarity to render the upper terminal of the latter positive. This potential on resistance 01 will cause a charging current'flo'w in the circuit including resistance 10 and condenser 89, which current in flowing through the resistance III will operate-to=render the upper terminal of the latter positive. This action willoperate. as previously explained, to

decrease the negative bias on the input circuit of valve 28, thus increasing the amplification factor or the latter and causing an increase in the alter nating potential induced in the transformer secondarywinding '41 and applied to the input circult of triode 21. ,Suchincrease in alternating potential on the input circuit oi. triodej'l effects a reduction in the alternating potential induced ment of contact it along resistance It required vto balance a given adjustment of contact II along resistance l3 and thereby the adjustment given valve 3 for such adjustment of contact l5 may be varied as desired, by moving contact 43 along the length of resistance 45. i

The adjustment of contact It and'valve 3 following an adjustment of contact l5, commonly known as a follow-up adjustment by those skilled in the art, is not permanent in its efl'ect because it requires for its continuance a potential drop across resistance W of a magnitude determined by the extent of departure of the slide contact H! from the position along resistance l3 corresponding to the temperature it is desired to maintain in furnace i. As the condenser 69 charges to the potential across resistance 51, the current flow through resistance It and the potential drop thereacross decreases. This results in an increase in the negative bias" on the input circuit oi valve It, and consequently; in a decrewe in amplification factor of valve 8 and a corresponding decrease in the alternating potential applied to the in the opposite direction to effect a closing adjust- 'ment of fuel valve 3 and positioning of contact contact l5 has returned to the desired position,

the adjustment of contact 16 being a true follow-up adjusment. That is to say, the extent of adjustment of contact IE to the right when the contact I5 is returned to its desired position will correspond to the extent of adjustment of contact l5.

Because of the resetting adjustments given the contact i6 following the initial departure of the furnace temperature from the control point, however, the total movement given the contact 16 to the left will have been greater than the movement of contact I5 to the left. As a result, when the contact i5 is restored to its desired position, the follow-up adjustment given contact ill to the right will be insuilicient to return the latter to the posi-- tion from which it initially departed. Thus, when the furnace temperature has been restored to the desired value the contact it will come to rest at a, position short of its initial position. With the contact l6 so displaced to the left of contact 51 along slidewire resistance M, a. potential drop will beproduced across resistance ill of the precise magnitude to exactly balance out the potential on condenser 69, and therefore, no potential drop will be then produced across resistance his; potential will then be produced in the transrorruer secondary winding M which is effective to cause iilrther operation or motor ii in the direction of initial operation and thereby an additio el opening adjustment of contact it to the leit. iiluch additional adjustment of contact it tends to restore the potential drop across the resistance it to the value required to maintain the followmp operation, but such further ad- 13. According to the present invention that rate is made sufficiently slow so that on an initial adiustment of contact l5, the total eil'ect of the charging of condenser 69 during the time of displacement of contact 15 from its desired position is only such as to compensate for the initial furlll and the amplification factor of valve iii will be determined solely by the potential drop across the cathode biasing resistance 712.

The arrangement of Fig. 1 will then stabilize, if

' no further change in the furnace operating connace characteristic change which caused the reditions has occurred meanwhile, with fuel being supplied to the furnace at a new and higher rate determined by the extent and duration of departure of the contact iii from its desired position. It will be apparent that for the case of decreased heat output of the furnace, the apparatus will eifeet a reduction in the supply or heat to the fur-- pace in a similar manner to thei'ebyreturn and maintain the furnace temperature at the desired value. o

It is thus seen that the adjustments effected by motor 5 are true compensating or resetting adjustments making it possible to maintain approximately the samefftirn ace temperature notwithstanding changes'in the furnace load which, for example, may result from substantial changes in the amount of material heated in the furnace. The rate at which such compensating adjustments are effected must be'suitably related to the constants, and particularly, the time lagconstant of the apparatus or process controlled, if hunting is to be avoided. In the arrangement of Fig. l the rate at which the fuel valve 3 is adjusted while the contact '55 is displaced from the control point may be controlled in a, number of ways, for example by adjustment of the capacity of the variable condenser. 69, by adjustment-of the value of resistance III, or by adjustment of an additional resistance which may be provided in series with condenser B9 and resistance 70 and. the

' parallel connected, resistance 61 and condenser 38, As one such means, I have illustrated the condenser 69 as being adjustable.

'In some applications it may be desired to utilize only the follow-up action of the control apparatus of Fig. 1, eliminating the resetting or compensating feature. When it is desired to so operate the apparatus, this may be accomplished by shunt-- ing out of the circuit the resetting condenser 69, as for example, by shunting the condenser 69 by a switch 69a. The apparatus will then operate as the corrective adjustment given fuel valve 3, on a departure of the furnace temperature from the desired value, to return that temperature to the desired value in the shortest possible time while minimizing the tendency to hunting. Parts in lv have been designed by the same reference numerals.

The arrangement of Fig. 2 includes a control network M2 for transmitting the movements of the movable element I of the pyrometer 8 to the fuel valve 3, The control network H2 comprises two branches in one of which the slidewire resistance I3 is connected and in the other of which the slidewire resistance I4 is connected. Resistances I I3 and H4 are alsoconncted in the branch including resistance I3, being disposed in that branch on opposite sides of the resistance I3 and the resistance I I4 being positioned to the right of resistance I3. The right end terminals of the two branches are connected to the grounded conductor 30. Alternating energizing current is supplied the control network I I2 from the secondary wind ing IQ of transformer H the primary winding I8 of which is connected to the. alternating current supply conductors L and L 7 In this modification the alternating potential produced between the slide contact I and the right end of resistance H4 is impressed on theprimary winding I I5 of atransforrner IIB having a secondary winding III. The transformer secondary winding H1 is connected to' the input circuit of an electronic valve H8 in series with a resistance l lfl across the latter of which an alternating potential is established by an electronic valve I20 under control of the slide contact I6. The transformer secondary winding Ill and the resistance '9 are so interconnected that the alternating potentialdrop across resistance I I9 is .o osed to that induced in secondary winding I H. The resultant of these alternating potentials is impressed on the input circuit of the electronic I value I I8.

Th manner in which an alternating potential is established across-resistance H9 under control of contact I6 and in opposition to thatinduced in transformer secondary winding III will now be described. This result is accomplished in Fig. 2 by rectifying the alternating potential produced across the portion of resistance I4 to the rightof contact I6 to establish a, unidirectional potential drop across the resistance 61 and utilizingthis potential drop to control the bias-potenmum an input circuit of-the valve 120. The unidirectional potential drop produced across re: sistance 61; inthis modification,'varis in magnitude in accordance with the position of contact I8 along slidewide resistance I4.

Fig. 2 which correspond exactly to parts of Fig.

nected through the resistance 61 to the grounded I conductor 30, which in turn, is connected tothe right end of resistance I4. This arrangement operates to establish a unidirectional potential across resistance 61 which gradually increases from zero to a maximum value as the contact I6 is adjusted from its extreme right to its extreme left positions.

The unidirectional potential so established across resistance 61 is applied to the input terminals of a filter I22 which comprises suitable resistance and capacitive components I22a, I221) and I220 and is employed for the purpose of smoothing out the potential drop produced across resistance 61. minal of the filter I22 is connected by a conductor I23, in which a resistance I24 and condenser 69 are inserted, to a switch arm I26.

' The switch arm I26 is movable between an upductor 3Il. Resistance I29 I30 are provided for the ary winding 43.

per contact I21 and a lower contact I28. In its upper position the switch arm I26 closes the circuit to the negative terminal of filter I22 through a resistance I29. and the grounded conis connected in an input circuit of valve I20. In its lower position the switch arm I26 closes the circuit to the negative terminal of filter I22 directly through conductor 30, resistance I29 being omitted from this closing connection. A condenser I30 is con-. nected between the grounded conductor 30 and the end of resistance I24 remote from the filter I22.

Condenser 69 is utilized in this arrangement. as in the arrangement of Fig. l, to produce the desired resetting or compensating adjustments of the system. The resistance I24 and condenser purpose of obtaining the heretofore referred to initially magnified fuel valve corrections on departure of the furnace temperature from the desired value, and operates to accomplish this result in a manner explained in detail hereinafter.

Electronic valve I20, to the input circuit of which the resistance I29 is permanently connected, is of the type commonly known as pentagrid converters and by way of examp e may be of the type known and sold commercially as the SL7. Valve I20 includes an anode, a cathode, a heater filament and five grids which are disposed between the anode and cathode. The grid nearest the anode is directly connected to the cathode and is utilized as a supressor electrode. The grid nearest the cathode is utilized as a control electrode and is connected through the resistance I29 to the grounded conductor 30 and through a cathode biasing resistance I3I shunted Energizing current is supplied the heater fllament of valve I20 from the transformer secondl Anode voltage is supplied the valve Whom the full wave rectifier 31 through a circuit which may be traced from the ositive terminal of The positive output terfilter 35.

Screen voltage is supplied valve I20 through a circuit which may be traced from the positive conductor 35 through a resistance I34 to the screen electrodes, and through a resistance I35 which isv shunted by a condenser I36 to the grounded negative conductor 30.

An alternating potential is applied to them-- termediate grid of valve I20 from the resistance 4 in the control network 2 through a circuit which may be traced from a contact I37 which is in slidable engagement with resistance II4, a conductor I38 in which a condenser I39 and a pair of adjustable resistances I40'and l4l are inserted in seriesto the grounded conductor 30 and therethrough to the right end terminal of resistance H4. The intermeriate grid of valve I20 is connected to the point of engagement of resistances I40 and-I4l. Resistance MI is provided with a condenser I42 in shunt therewith.

Thus, the valve I20 is provided with two input circuits on the first of which a unidirectional bias potential is arranged to be impressed from the resistance 61, and on the second of which an alternating potential is arranged to be impressed from the control network resistance H4, and on both of which a unidirectional bias is impressed from the cathode biasing resistance I3l.

According to this modification of my invention the amplification factor and therefore, the gain of valve I20 is a predetermined value when no bias potential in addition to that established across biasing resistance I 3| is impressed on the input circuit-first mentioned, as for example when switch I 26is in engagement with contact I20, whereby the alternating component of potential drop produced across the anode resistance I33 is a corresponding predetermined value. This alternating potential component is impressed through a condenser I43 on the resist:

'ance II9 which is connected in the input circuit of valve IIO through a circuit which may be traced from the anode of valve I20 through a conductor I44 in which the condenser I43 and resistance 9 are connected in series to the.

grounded conductor 30. Thus, the condenser I43 and resistance III! are connected in parallel to the-impedance comprised of the cathode biasing-resistance I 3| and the anode to cathode resistance of valve I20.

The phase of the alternating potential impressed on the second mentioned input circuit of valve I20 is so related to the alternating potential induced in the transformer secondary winding II I that the alternating potential established across resistance H9 is exactly 180 out of phase with the alternating potential induced in transformer secondary winding Ill. The series connected condenser I39 and adjustable resistances I40 and I are provided in the second mentioned input circuit of valve I20 to permit shifting of the phase of the alternating potential applied to this input circuit'of valve I20 as is required to effect such exact phase opposltion of the alternating potentials produced across elements II I and I I9.

' When the bias potential on the first mentioned input circuit of valve I20 is varied, as it may be when the switch I26 is in engagement with contact I21 and a charging or discharging current is flowing to or from the resetting condenser 59 through resistance I29, the amplification factor of valve I20 is changed an amount proportional to the change in bias potentiaL- This change in amplification factor of valve I20 operates to produce a corresponding variation in the alternating component of the potential drop across resistance I33 and therefore, in the alternating potential impressed on the resistance I I9. The variation in amplification factor of valve I20 is utilized, as is the amplification factor change 0f valve 26 in the arrangement of Fig. 1, to effect the desired followup and resetting adjustments of the system, and is also employed as later explained to produce the initially magnified fuel valve adjustments.

The resultant of the opposed alternating potentials established across the transformer secondary winding Ill and resistance H9 is applied to the input circuitof electronic valve ll8 wherein it is amplified and the amplified quantity is impressed on the input circuits of the two triodes I 92 and 93 contained within the electronic valve I31. Valve I I8 is a pentode having an anode, suppressor, screen .and control electrodes, a cathode and a heater filament to the latter of which energizing current is supplied by the transformer secondary winding 43. Anode voltage is supplied valve ll8 from the rectifier 31 through a circuit which may be traced from the positive terminal of filter 35 through a conductor I45, a resistance I45, the anode to cathode resistance of valve II8, a cathode biasing resistance I41 which is shunted by a condenser I49, and conductor 30 to the negative terminal of filter 35. .Energizing voltage is supplied the screen electrode through a circuit which may be traced from conductor I45 to a resistance I49 to the screen and from the latter to 4 I they will neutralize each other. When one or the other of the alternating potentials predominates, however, a resultant alternating potential of corresponding phase will be applied to the input circuit of valve'l l8 to thereby establish an alternating potential of corresponding phase but of in.- creased amplitude across resistance l 46.

. The alternating potential so established across resistance I45 is impressed simultaneously and equally on both of the control electrodes of triodes 92 and 93 through a condenser l5l and a resistance I52. The resistance I52 effectively limits the extent to Which the controlelectrodes of triodes 92 and 93 may be driven positive with respect to their associated cathodes. A condenser I53 is desirably connectedin shunt to resistance I52 for filtering out harmonic potential components which may appear across resistance I52 and otherwise tend to swamp out the regulating potential applied to input circuits or triodes 92 and 93.

Anode voltage is applied to the output circuits of triodes 92 and 93 from the alternating current supply conductors L' and L through a transformer .l54 having a line voltage primary winding I55 which has its terminals connected to the supply conductors L and L and also having a. center tapped secondary winding I55. The

anode of triode 92 is connected to the left end terminal or the transformer secondary winding I56 and the anode of triode 93 is connected to the right end terminal or the transformer secondary winding I56. The cathode of triode 92 is connected through a cathode biasing resistance I58 and a limit switch I69 to the terminal I60 of one phase winding I6I of a two phase reversible rotating field motor I62. The cathode of triode 93 is connected through a cathode biasing resistance I63 and a limit switch I64. to the terminal I66 of motor winding I 6|. The other terminal I65 of motor winding IN is connected to the center tap on the transformer secondary winding I56. Thus, the triodes '92 and 93 are utilized-in this embodiment of my invention for supplying energizlng current to the phase winding I6I of motor I62.

The motor I62 is preferably so constructed that the impedance of the winding I6I is of the proper value to match the impedance of the anode circuits of triodes 92 and 93 when the motor is oper-- ating in order to obtain the most efficient operation. Preferably, the; motor'is so constructed that it has a high ratio of inductance to resistance, for example, of the order of 6-1 or from 8-1 at the frequency of the energizing current supplied to it. This provides for maximum power during the running condition of the motor with the least amount of heating,and also provides a low impedance path for braking purposes.

Motor, I62 is provided with a second phase wind- Ell triode 92 is rendered positive with respect to said center tap. In the second half cycle of that alternating potential, the anode of triode 93 is rendered positive. Accordingly, the triodes 92 and 93 are arranged to conduct on alternate half cycles of the alternating current supplied by the supply conductors L and L When no alternating signal or grid bias is impressed upon the control electrode of the triodes 92 and 93 pulsating unidirectional current of twice the frequency of the alternating voltage supplied by conductors L and L is impressed on the motor winding I6I. When thus energized the motor is not effectively urged to rotation in either direction, but remains at rest. Due to the relatively high direct current component of the current then flowing through the motor winding I6I the core structure of the motor I62 tends to become saturated whereby the inductive reactance of the motor winding l6I is relatively small. The con-' denser I68 in shuntto the winding I6I is so chosen that the condenser and motor winding ing I66 to which energizing current is supplied from the alternating current supply conductors L and L through a condenser I61. The condenserIG'I is so selected with respect to the inductance of motor winding I66 as to provide a series resonant circuit having a unity power fadtor. By virtue of the series resonant circuit, the total impedance of the motor winding I66 is substantially equal to the resistance of the winding, and since this resistance is relatively low, a large current flow through the winding B66 is made possible. This permits the attainment of maximum power and torque from the motor I62 with the least amount of heating. In addition, the current flow through the motor winding I66 is in phase with the voltage of the alternating current supply conductors L and L because of the series resonant circuit. The voltage across the motor then provides a parallel resonant circuit. This saturation of the core structure of the motor I62 operates to exert an appreciable damping effect on the motor rotor, or in other words, an efiect tending to prevent rotation of the rotor. Conseuuently, if the motor rotor has been rotating,

saturation of the motor core structure operates to quickly stop the rotation. When an alternating potential is impressed on the control electrodes of triodes 92 and 93, the

magnitude of the pulses of current flowing in the anode circuit of one triode 92 and 93 will beincreased while the magnitude of the pulses of cur rent flowing in the anode circuit of the other triode will be decreased. Accordingly, the pulses of v unidirectional current supplied to the motor wind- I ing I6 I during the first half cyclewill predominate over those supplied the motor during the second half cycle. Which anode current will be. increased depends upon whether the signal voltage is in phase or 180 out of phase withthe voltage of the supply conductors L and L Such energization of the motor winding I6I operates to introduce therein an alternating component of current of the same frequency as that supplied by the alternating current supply conductors L and L This alternating component of current will either lag by 90 or lead by 90 the" alternating current flowing through the motor wind-I v ing I 66 depending upon which of the triodes 92 and 93 has its anode current increased by the prevailing alternating potential on the control electrodes,

' and with either phase relation the twocurrents 93 through thecircuits previously traced. A 0011- denser I 68 is connected in parallel with the motor winding IN and is so chosen as to provide a parallel resonant circuit having a unity power factor.

This parallel resonant circuit presents a relatively high external impedance and a relatively low local circuitv impedance. The relatively high external impedance isapproximately the same as the impedance of the anode circuits of the triodes 92 and 93, and accordingly, provides efficient operation. The relatively low internal circuit impedonce approximates the actual resistance of the winding I6I and since this resistance is relatively low, the impedance of the local circuit isalso relatively low. For the first half cycle of the alternating poten-' tial produced across the terminals of the. transformer secondary winding I66, the anode of the produce a magnetic field in the motor core structure which rotates in one direction or the other.

depending upon said current phase relation and eifects rotation of the motor in the corresponding direction. Moreover, when the motor winding I6I is so energized the direct. currentpomponent of current flowing therein is decreased, and .consequently. the saturation of the motor core structure is decreased with the result that the rotor damping efiect is reduced.

This general arrangement for operating a reve'rsible rotating field motor isdisclosed and claimed in the Wills app'licationSerlal No. 421,173

which has been-referred to hereinbefore. The

. provision of the limitswitches- I69 and I64 in the cathode circuits of triodes 92 and 93 is novel with me, however,v and comprises a part of the present invention. These limit switches are op-' erated by a suitable member rigid with the shaft of motor I62 and are arranged to interrupt the energizing circuit of triode 92 or 93 to. the motor phase winding I6I when the motor ha's'operated to the extreme limits of adjustment of the fuel valve 3.

In Fig. 2 the control point, or in other words the furnace temperature it is desired to maintain, can be established at any point on the instrument slidewire resistance I3. A prerequisite for the establishment of the control point at any desired position along the resistance I3 is that the alternating potential induced in the transformer secondary winding H1 is equal and opposite to the alternating. potential produced across resistance H9 when there is no current flowing in resistance I29. This condition is satisfied in the arrangement of Fig. 2 by throwing the switch arm I26 into engagement with the contact I28, the Stabilize positionof the switch, and adjusting the contact I31 along the length of resistance I I 4 until the alternating potentials across the elements H1 and I I9 are equal and opposite at the desired control point. The establishment of this condition will be indicated by the fact of the motor I62 remaining at rest. Both of the limit switches I59 and I64 must be closed when making this adjustment, however. The switch arm I26 may then be moved into its Run position, in engagement with the contact I21.

By way of illustration it is noted that when the voltage of-the alternating current supply conductors L 'and L is 110 volts and is of a frequency of 60 cycles per' second, the valve I I8 may be of the type known commercially as a 6J7 type, the valve I2I may be a type '7F'7,'the valve I26 may. be a type.6L'7, the "valve 81 may be a type 7N7,' the transformer H6 may be a 1-1 ratio Thordarson type 57A39 transformer, and correspondingly suitable values for the various circuit constants may be as noted in the following table.

Reference numeral Quantity Suggested value 55 ohms. 50 ohms.

50,000 ohms. 25 microiarads. 110,000 ohms. l microfarads.

,000 ohms. 4 microI'al-ads. 10 microfurads.

150,000 ohms.

0.5 microfarads.

0.05 microfarads.

1,500 ohms.

150 ohms.

550 volts.

(275 volts from center tap to each end terminal.)

1 microfarad.

o Terminal voltage....

In Fig. 2 the initial effect of a change in fur- Condenser -nace temperature and consequent movement to theright or left of the instrument contact I! from the control point is to changev the altercisely that needed to produce a change in the unidirectional potential drop across resistance 61 and consequently a change in the potential drop across resistance I29 of the proper amount to change the amplification factor of valve I20 as required to make the alternating potential drop across resistance H9 exactly equal to the new value of potential induced in th transformer secondary winding I I1.

Specifically, on a decrease in the furnace temperature and a resultant adjustment of contact I5 to the left, the motor I62 will be initially operated in the proper direction and the proper amount to produce a corresponding valve opening adjustment and adjustment of contact I6 to the left. Similarly, an increase in furnace temperature and subsequent adjustment of contact I5 to the right will operate to energize motor I62 for rotation in the direction and to the extent to effect a corresponding valve closing adjustment and adjustment of contact I6 to the right.

Such system rebalancing or follow-up adjustments of the motor I62 are not continuous in their efiect when'the instrument contact I5 is displaced from the control point, however, but gradually diminish, and if no further adjustments were made, would disappear entirely. This phenomenon occurs as a result of the connection of the condenser 69, as shown, and is employed to effect the desired resetting adjustments of the control apparatus. In particular, as the condenser 69 gradually charges to the new value of potential drop produced across resistance 61 by the follow-up adjustment of contact I6 along resistance I4, the current flow and thereby the potential drop throughv resistance I29 gradually decreases. This action operates to tend to restore the amplification factor of valve I26 to the value determined solely by the cathode biasing resistance I3] and condenser I32, and therefore, the alternating potential established across resistance H9 tends to be restored to its original value. Thus, the induced alternating potential in the transformer secondary winding-H1 again becomes effective to energize the motor I62 for further rotation in the same direction. The motor I62 and contact I6 will therefore creep along in the direction of initial movement as long as the contact I5 is displaced from the con- .trol point to tend to maintain a steady flow of current through resistance I29 as required to make the alternating potential drop across resistance H9 equal to that established across transformer secondary winding H1, and to give additional adjustments to the fuel valve 3. The charging or discharging action of the condenser 69 is continuous-as long as the contact I5 is displaced from the control point, and hence the rebalancing adjustment of the contact. I6 is continuous until the contact I5 hasreturned to that I position.

The rate at which the condenser 69 charges and discharges is determined by the extent of departure of the contact l5 from the control point, the capacity of condenser 69, the magnitude of the resistance I29, and also the values of the other circuit constants connected in series with condenser 69, resistance I29 and resistance 51. That rate, as in the arrangement ofFig. 1 is desirably slow so that on an initial adjustment of the contact I5 the total effect of the charging or discharging of condenser 69 during the time of displacement of contact l5 from the control point is only such as to compensate for the asvacie initial furnace characteristic change which caused the change in furnace temperature.

When the relationship between the gain of the valve I20 and the unidirectional bias potential impressed thereon from the resistance I29 is linear, thefollow-up action of the contact I6 along resistance I4 following a furnace temperature change is linear irrespective of the position of the instrument and valve contacts along their respective slidewire resistances, and in addition, the rate at which the resetting adjustments are produced is proportional to the deviation of contact I from the control point. To obtain such linearity it is noted that the rectifier valve Ill must have the characteristic that the unidirectional potential drop produced across resistance B'l is linear with respect to the alternating potential impressed on valve I2I from the slidewire resistance It. Over the operating range of the apparatus a type 7F? valve generally exhibits such a characteristic. 7 I

In order to maintain the eifectiveness of a follow-up adjustment of the motor I62 a definite bias voltage depending on the extent of departure of the instrument contact I5 from the control point, must be maintained across resistance I29. 7 This bias voltage can only be maintained by a constant current flow through resistance voltage across resistance I29, and therefore, also proportional to the extent of deviation of the contact I5 from the control point. I

The initial magnification of the corrective ad- .justment of the fuel valve 3 referred to hereinbefore is obtained by temporarily reducing the I effectiveness of the follow-up or rebalancing adjustments of the contact I5 along slidewire resistance I4 whereby a magnified adjustment of the contact I6 along resistance I4 is effected.

Such temporary reduction in the efiectiveness of rebalancing adjustments of the contact I6 is produced by resistance I24 and condenser I311. The connection of resistance I2! and. condenser I as shown initially operates to subdue or minimize changes in the potential tending to be impressed on the series connected condenser 69 and resistance I29 following a given change in position of contact I6 along resistance I4, and thereby in potential across resistance 51. Such subduing of the potential impressed on condenser 69 and resistance I28 is caused by the resistance I24 and condenser I30 cooperating to delay for a suitable interval the charging of the latter to the potential appearing at the output terminals orfilter I22. Accordins y, contact I8 will initially be given a magnified adjustment'in efleoting the iollow-up or rebalancing adjustment of network II2. l

- Stated differently, during the initial stages of i the follow-up operation only a fraction of the eifect of the follow-up adjustments of the contact I6 along slidewire resistance I4 will be effective to change the potential drop across resistance I29 and thereby the gain of the valve I20. The magnitude of this fraction depends upon the value of resistance I24 and condenser I30. Therefore, alar'ger follow-up adjustment of contact I6'is required initially to cause the alternat- I 29 which, in turn requires that thepotential 7 rate of the change in the condition being consult the change in potential drop created across' resistance IIS will overbalance the change in potential across secondary winding llll caused by the departure of contact it from the control point, and consequently, the motor BS2 will be energized for rotation in the reverse direction and operate to return the contact it to its true follow-up position, thus reducing the magnification of adjustment oi contact it and thereby of valve 3 to unity.

Similarly, on a return adjustment oi the instrument contact iii to the control point, the contact It will immediately be given a magnified thereafter the magnification in the adjustment will gradually be removed.

In the foregoing explanation of the operatio of the arrangement of Fig. 2, the condition considered has been that wherein the contact 85 is given an adjustment away from'the control point and then remains stationary until returned to the control point. It will be apparent, however,-

that in practice the adjustments of the contact I5 along slidewire resistance it are ordinarily gradual. If the contact I5 is adjusted slowly and continuouosly in the same direction, the tendency for the magnification in the adjustment of contact I6 and valve 3 to build up to its maximum value will be decreased by the potential change which is permitted to take place on condenser I30, and accordingly, the magnification will assume an-intermediate value determined by the rate of adjustment of contact I5. The magnification of the adjustment of contact I8 will thus vary in proportion to the rate of adjustment of contact I5 and will be larger when terms of rate of change since the magnitude of the magnification obtained is proportional to the trolled.

'The effect of such operation is to permit the initial corrective control effect in the supply of heat to the furnace I to be much larger than would be possible otherwise without resulting in overshooting and consequent hunting. This advantageous result is obtained because the greater part of the corrective effect is applied during the time of increasing departure of the furnace temperature from the desired value, and

ing potential across resistance II! to match that induced in the transformer secondary winding .I I1 than would otherwise be-needed forthis puris removed at 'a suitable time before the furnace temperature has returned to the desired value to avoid overshooting and consequent hunting.

The duration and effective value of the magniflcation in the control etfect produced by the resistance I2! and condenser I30 and their associated elements may be varied by adjustment of either or both of the elements- I24 and I30. To this end the elements I24 and I30 have both been shown as being variable. The effect of adjustment of resistance I24 or condenser I30 is to vary the time required to charge the condenser I30 to the potential at the output terminals of filter I22 following a change in the latter potential.

It is noted that the initial magnified adjustments of the contact I6 and valve 3 may be limited to a desired value, ona given adjustment of contact I from the control point, by

making the speed of adjustment of contact I6 suitably slow, as by means of the gearing 4 between the motor I62 and the valve 3, the time of adjustment of contact I6 operating to produce this result.

In Fig. 3 I have illustrated, more or less diagramatically, a modification of the control arrangement of Fig.2 in which the follow-up and resetting functions have been separated so as to be independently variable. Separate reversible motors and electronic amplifiers have been provided for accomplishing each function. This modified arrangement, in addition, makes possible a substantial reduction in the rate at which the resetting adjustments of the fuel valve 3 are effected over the slowest rate obtainable with the arrangement of Fig. 2.

With the control arrangement of iig. 2 the rate at which the resetting operation produced and the follow-up or throttling lge may be represented by the relationship given below. By throttling range is meant the extent of adjustment of contact I5 required to effect a complete adjustment of valve 3 from its fully opened to its fully closed position.

Reset rate=%% b Ev Ei Throttling ran'ge= Where D Deviation of contact Iii from control point Ei=Voltage between contact I5 and right end of resistance ti l.

C=Capacitance in microfarads of condenser I30.

R:Resistance in ohms of resistance I2 i.

Ev voltage between contact I6 and right end of resistance M.

a and b=constants.

From these relationships it will be evident that after C and R have been increased to their practical limits in decreasing the reset rate that further reduction in the reset rate must be accomplished by either decreasing Ei or increasing Ev. Reduction in the reset rate by varying E1 or E1) is not desirable, however, because such reduction is obtained at the expense of increasing the throttling range. This problem is overcome in the arrangement of Fig. 3 by means of reducing the value of the constant a. Further reduction in the reset rate may be obtained in this manner without affecting the throttling range. To vary the value of the constant a, however, it is necessary to separate the follow-up and resetting functions of the control arrangement as will become apparent from the following detailed explanation of the arrangement of Fig. 3.

In this embodiment of my invention the move ments of the movable element II] of the pyrometer ll are transmitted to the fuel valve 3 by means including a control network I68 which, as shown,

is a self balancing type Wheatstone bridge network. Alternating energizing current is supplied the bridge network I68 from the alternating current supply conductors L and L To this end two opposite bridge conjugate points are connected by conductors 20 and 2I to the terminals of the transformer secondary winding I9. One of these bridge energizing conjugate points is the point of engagement of resistances I69 and I II, the latter of which is variable, and the other of these bridge energizing conjugate points is the point of engagement of resistances I10 and H2. Resistances' I69 and I70 are fixed in magnitude while the resistances Ill and I12 are variable. Variable resistances HI and H2 are provided for the purpose of effecting the desired resetting adjustments of the system.

The other two opposite bridge conjugate points are connected by conductors I13 and I'M to the input terminals of the electronic valve II8. One of the last mentioned bridge conjugate points is the point of engagement of slide contact I5 with the slidewire resistance I3 which is connected in the network I68 between the resistances I69 and III). The slide contact I5, in this modification, as in the arrangements of Figs. 1 and 2, is adjusted along slidewire resistance I3 in accordance with the changes in furnace temperature to which the pyrometer 8 is responsive. The other of the last mentioned bridge conjugate points is the point of engagement of contact I6 with the slidewire resistance I4. It will be noted that this point of engagement is grounded, being connected by conductor I'M to the grounded conductor Bil. Resistance Ill is connected in the network I68 between the variable resistances HI and I12. Contact It is adjusted along the length of the resistance M by the reversible rotating field motor I62 which is operatively connected to the contact I6 and also to the valve 3, as in the Fig. 2 arrangement, through the gear mech anism l.

When the furnace temperature is at the value it is desired to maintain, the potential of the contact i5 is exactly the same as the potential of the contact I6. Contact I 5, as illustrated, is connected by the conductor I13 to the control electrode of valve I I S and contact I6 is connected by the conductor I74 and the grounded conductor 36 to the cathode of valve II8. With the furnace temperature at the control point, therefore, no alternating potential willbe impressed between the control electrode and cathode of valve M8.

Upon deviation in the furnace temperature and resultant movement of the contact I5 from the control point, however, an alternating potential of one phase or of opposite phase, depending upon the direction of the furnace temperature change, will be established between the contacts I5 and I6. This alternating potential is amplified by the valve IIB and the amplified quantity is utilized, as explained in connection with the arrangement of Fig. 2, to control the relative conductivities of triodes 92 and 93 and thereby the energization of the motor I62 for operation in one direction or the other. Motor I62, in its operation, actuates contact IS in the direction along the length of resistance I4 to reduce to zero the alternating potential between the contacts I5 and I6 and when this has been accomplished assumes a condition of rest. In this manner the contact I6, and thereby the fuel valve 3, is given a follow-up adjustment in the direction and an amount corresponding to the asvasia direction and extent of adjustment of the contact I5, If no further adjustment of the bridge network resistance were made, the motor I62 would then remain at rest.

According to this embodiment of my invention, however, the motor I92, is caused to produce additional adjustments of the contact I6, and therefore of the fuel valve 3, in the same direction so long a the contact I is displaced from the control point. To this end the alternating potential established between the contact at and the right .end of resistance I'lll is utilized to regulate the rotation and direction of rotation of a reversible rotating field reset motor I75. The shaft of motor I15 is connected through suitable gear means I16 to slide contacts In and I'lii which are in engagement with resistances I'll. and I12, respectively, and so arranged that optrode being connected to the point of engage ment of the latter elements. Energizing current is supplied the heater filament of valve I99 from the transformer secondary winding 33. The input circuit of valve I89 includes in addition to the transformer secondary winding II! the cathode biasing resistance I62 and shunt condenser I83 and the resistance llll across whichan alternating potential 180 out of phase with that induced in transformer secondary winding Ill is established by the electronic valve 9% which, in turn, is controlled in accordance with the movement of contact it along slidewire resistance 86.

Valve we may be of thesame type and is energizeol in the same manner as the correspondingly identified valve of Fig, 2 and has an alternat ing potential of predetermined magnitude ixn= pressed on theinput circuit includingthe control grid furthest from the cathode from the secondnetwork I68, and therefore, to produce an altemating potential between the bridge contacts The-direction of operation of motor I15 is so relatedto the direction of deflection of contact: is from the control point that. this additional unbalancing of the bridge network by variation of resistances Ill and. I12 is in the same r direction as that initiallycaused by .the movementof contact I5. .As a result of such-additional unbalancing of the bridge network I69, the motor I92 operates to produce further network rebalancing adjustments of the contact I6 in the same direction and corresponding additional adjustments oi the Iuelvalve 9. v 4

The manner in which the operation of the reset I motor I15 is controlled in response to deflections of the contact I5 from the controlpoint is essentially thesame as themanner in which the motor I92 of the Fig. 2 arrangement is controlled to produce the resetting iunction of that system. Certain iundamentaliand important ditft'erences exist between the two arrangements, however, which permit of the attainment of improved operation by means of the arrangement 01' Fig. 3

, under certain conditions. These differences will become apparent as the description of the Fig. 3 embodiment proceeds.

In the Fig. 3 arrangement, similarly to that of 1 Fig. 2, the alternating potential between the contact I5 and the right end-terminal of resistance I10 is impressed through conductors I" and I19 on the primary winding H5 ortransformer H9 and the alternating potential induced in the transformer secondary winding III is impressed on the input circuit of an electronic valve I99. Valve I99 may be/oi the same type as valve H9 and is supplied with anode em tgizing voltage from the full wave rectifier 31 and filter 99 through a circuit which may be traced from the positive terminal of filter 39 through conductor I45, a. resistance I9I, the anode to cathode resistance of valve I99, a cathode biasing resistance -I82-which is shunted by a condenser I93, and the grounded conductor 30 to the negative terminal of filter 39. Screen voltage is supplied valve I90 trolled in accordance with the magnitude of the unidirectional potential created across the re sistance I29, and'impressed on the control electrode nearest the cathode, as the contact It is adjusted along the length of resistance It. This unidirectional potential across resistance I29 is derived through the filter I22 and the condenser 99 from the potential drop produced across resistance 91 by the flow of current through the latter from rectifier It I In order to decrease the rate at which the resetting function of the apparatus is obtained, however, the alternating potential established between the contact I9 and the left and or resistance I4 isnot directly impressed on the output circuit of rectifier valve I2I as in the Fig. 2 arrangement, but is first amplified by means or an electronic valve I9I.- Valve I9I is atriode havfilament elements and receives anode energizing voltage from the full wave rectifierWl and filter from the positive conductor I49 through a resistance I94 and a condenser I95, the screen elec- 99 through the positive conductor I45 and the negative grounded conductor 99. As shown, a.

cathode biasing resistance I92 which is shunted by a condenser I93 is connected between the cathode of valve I9l and the grounded'conductor 30 and an anode, resistance I94 is connected between the anode and the positive conductor I45. Energizing voltage is supplied the heater filament of valve I9I from the transformer secondary winding 43.

The input circuit of valve I9I may be traced from the control electrode through a conductor I99 to the left end of resistance I4, the portion of resistance I4 to the left of contact I9, contact I9, conductor I14, the grounded conductor 90,

and the parallel connected resistance I92 and cathode of valve I9I is connected by the parallel connected elements I92 and I93. Thus, the amplified fluctuations in potential of the anode of valve I9I caused by the alternating potential between the left end of resistance I4 and contact I6 are impressed on the rectifier valve I2I to produce a unidirectional potential drop across resistance 61 having a magnitude corresponding to the position of contact I6 along resistance I4.

This potential drop across resistance 61 is impressed on the circuit including the filter I22, the condenser 69 and the resistance I29 to control the gain of the electronic valve I20 and thereby the magnitude of the alternatingpotential drop impressed on the resistance H9. The condenser 69 operates in this arrangement as in those previously described to gradually diminish the cur rent flow through resistance I29 and thus the unidirectional potential drop created across the latter as a result of a given adjustment of contact I6 along resistance I4, thus requiring continuous uniform movement of contact- I6 along resistance I4 to maintain a steady unidirectional current flow and consequent steady unidirectional potential drop across resistance I29. The magitude of the unidirectional potential drop across resistance I29 required to cause the alternating potential across resistance IIS to exactly balance the alternating potential induced in transformer secondary winding I I1 is determined by the magnitude of the latter alternating potential, and therefore, upon the extent of departure of the contact I5 from the control point. Consequently, the rate of adjustment of contact I6 along resistance I4 needed to make the alternating potential drop across resistance I I9 balance that in duced in transformer secondary winding II I is determined by the departure from the control point of the contact I5.

The output circuit of valve I80 is coupled by a condenser I91 and a resistance I98 to the input circuits of a pair of triodes I99 and 206 which are contained within a single bottle 2M and may be identical to the triodes 92 and 93. The output circuits of triodes Hand 2% are energized from the center tapped secondary winding 262 of a transformer 203 having a line voltage primary winding 2M connected to the alternating current supply conductors L and L In particular, the anode circuit of triode 599 may be traced from the left end of winding 202 to the anode, the oath ode, a biasing resistance 205, a limit switch 206, and one phase winding 201 of motor I to the center tap on winding 202. Similarly, the anode circuit of triode 200 may be tracedfrom the right end of winding 252 to the anode, the cathode, a biasing resistance 208, a limit switch 209, and the motor phase winding 291 to the center tap on winding 202.

Motor I15 is also provided with a second phase winding 2Ifil which is connected to the alternating current supply conductors L and L through a suitable condenser 2| I. As illustrated, a condenser 2 I 2 is provided in shunt to the motor winding 201.

The design and construction of the motor I15 as well as its operation under control of triodes 599 and 200 may be exactly the same as that of motor I52 which has previously been described in detail in connection with Fig. 2, and therefore, further description of the operation of motor I15 is not believed necessary. Suffice it to say that when the alternating potential across resistance II9 exactly balance out the alternating potential induced in transformer secondary winding II1, 'the motor I15 remains at rest, but when either of said alternating potentials predominates, the motor I15 will be actuated for rotation in a corresponding direction to effect such adjustment of variable resistances HI and I12 in the bridge network I68 as required to cause additional resetting or compensating adjustment of contact I6 in the direction of its initial movement to maintain the bridge network I68 in a balanced state.

As those skilled in the art will recognize the provision of the amplifier valve I9I makes it possible to decrease the rate at which such resetting or compensating adjustments are effected by a factor proportionate to the gain of the amplifier valve I9I below the slowest rate obtainable without the valve I9 I. Such reduction in the reset rate, moreover, is accomplished without affecting the throttling or follow-up range of the system, being accomplished instead by reducing the value of the constant a in the relationship hereinbefore noted as representing reset rate. Mathematically, the amplifier stage including valve I9I functions to reduce the value of the constant a.

While I have illustrated several embodiments of electrical apparatus according to my invention for automatically controlling the temperature of a furnace, it will be apparent to those skilled in the art that my invention is not limited to'such use and may, for example, be employed with equal facility for regulating pressure, flow, liquid level, and other variable characteristics. Such adaptation of my invention may be readily effected merely by operating the contact I5 of any of the forms of my invention which have been illustrated in the drawings in accordance with the pressure, flow, liquid level, etc., variations.

While in accordance with the provisions of the statutes, I have illustrated and described the best forms of embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the forms of the apparatus disclosed without departing from the spirit of my invention, as set forth in the appended claims and that in some cases certain features of my invention may be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is as follows: i

1. A system for the automatic control of a variable characteristic having adjusting means for controlling the value thereof comprising a normally stabilized alternatin current energized control network, means to render said network unstable proportionally to deviations of the characteristic from a desired value, and electrical means including electrical reactance means as sociated with and. actuated in response to insta bility of said network to initially operate said adjusting means an amount corresponding to the extent of deviation of the characteristic from the desired value, said electrical reactance means operating thereafter to modify the response of said electrical means to the state of stability of said control network to cause additional operation of said adjusting means in the direction of its initial adjustment.

2. A system for the automatic control of a variable characteristic having adjusting means for controlling the value thereof .comprising a normally stabilized alternating current energized control network, means to render said network means including electrical capacitance means associated with and actuated in response to in.- stability of said network to initially operate said adjusting means an amount corresponding to the extent of deviation of the characteristic from the desired value, said electrical capacitance means operating thereafter to modify the response of said electrical means to the state of stability of said control network to cause additional operation of said adjusting means in the direction of its initial adjustment.

3. A system for the automatic control of a variable characteristic having adjustin means for controlling the value thereof comprising a normally ,stabilized alternating current energized control network, means to render said network unstable proportionally to deviations of the characteristic from a desired value, and electrical means including electrical reactance means and a variable gain electronic valve controlled thereby associated with and actuated in response to instability of said network to initially operate said adjusting means an amount corresponding to the extent of deviation of the characteristic from thedesired value, said electrical reactance means operating thereafter to vary the gain of said variable gain electronic valve to modify the response of said electrical means to the state of stability of said control network to cause additional operation of said adjusting means in the direction of its initial adjustment.

4. A system for the automatic control of a variable'characteristic having adjusting means controlling the value thereof comprising a normally stabilized alternating current energized control network, means to render said network unstable proportionally to deviations of the characteristic from a desired value, a reversible electrical motor to adjust said adjusting means, and electrical means including electrical reactance means and a variable gain electronic valve controlled-thereby associated with and actuated in response to instability of said network to initially energize said motor of operation to adjust said network to its stabilized state and to simultaneously operate said adjusting means an amount corresponding to the extent of deviation of the characteristic from the desired value, said elec-' trical reactance means operating thereafter to vary the gain of said associated variable gain electronic valve as long as said characteristic is deviated from the desired value to modify the response of said electrical means to such restabilizing adjustment of said network to thereby additionally energize said motor'to effect additional .restabilizing adjustments of said network and additional operation of said adjusting means in the direction of its initial adjustment.

7. A system for the automatic control of a variable characteristic having adjusting means for controlling the value thereof comprising a for controlling the value thereof comprising a normally stabilized alternating current energized control network, means to render said network unstable proportionally to deviations of the characteristic from a desired value, a reversible electrical motor to adjust said adjusting means, andelectrical means including electrical reactance means associated'with and actuated in response to instability of said network to initially energize said reversible motor for operation as required to operate said adjusting means an amount corresponding to the extent of deviation of the characteristic from the desired value, said electrical reactance means operating thereafter to modify the response of said electrical means to the state of stability of said control network to cause additional energization of said motor for further operation of said adjusting means in the direction of its initial adjustment.

5. A system for the automatic control of a variable characteristic having adjusting means for controlling the value thereof comprising a normally stabilized alternating current energized control network, means to render said network unstable proportionally to deviations of the characteristic from a desired value, and electrical mean including electrical reactance means associated with and actuated in response to instability of said network to initially adjust said network to its stabilized state and to simultaneously operate said adjusting means an amount corresponding to the extent of deviation of the characteristic from the desired value, said electrical reactance means operating as long as said characteristic is deviated from the desired value to modify the response of said electrical means to-such restahilizing adjustment of said network to thereby effect additional restabilizing adjustments of said network and additionaloperation of said adjusting I -means in the direction of its initial adjustment.-

6. A system for the automatic control of a variable characteristic having adjusting meansfor normally stabilized alternating current energized control network, means to render said network unstable proportionally to deviations of the characteristic from a desired value, and electrical means including electrical reactance means associated with and actuated in response to instability of said network to initially operate said adjusting means an amount-corresponding to the extent of deviation of the characteristic from the desired value and to restore said network to its stabilized state, said electrical reactance means operating thereafter to diminish the effectiveness of such restabilization of said network on said electrical means to thereby effect additional operation of said adjusting means in the direction of its initial adjustment.

8. A system for the automatic control of a variable characteristic having adjusting means for controlling the value thereof comprising a normally stabilized alternating current energized control network having a control slidewire resistance and slide contact and a follow-up slidewire resistance and slide contact, means to adjust said control slide contact along said control tact along said follow-up slidewire resistance, electronic amplifier means to regulate the operation of said control means, said electronic amplifier having output terminals to which saidcontrol means are connected, means to control the operation of said electronic amplifier by said first mentioned alternating potential, means to derive an additional alternating potential to control the operation of said electronic amplifier including a variable gain electronic valve included in said electronic amplifier and having output terminals from which said additional alternating 

